Preparation and photophysical properties of organic fluorescent polymers and their nanoparticles

New classes of stable organic florescent polymer nanoparticles were prepared from the corresponding tetrahydrofuran (THF) solutions, upon slow evaporation of solvent under a mild vacuum. Uniform spherical nanospheres having mean diameter of ? 500–700 nm, showed enhanced fluorescence emission in solution, but it diminished when it is destroyed using an organic base. These end‐capped fluorescent polymers are highly stabilized in the excited states at higher concentration in solution, leads no more intra‐ and intermolecular interactions among the excited molecules, because of the absence of free functional groups. The significant enhancement in fluorescence emission was attributed to the high level of molecular stacking in the fluorescent nanoparticles, when compared with micron‐sized isolated powder sample. In addition, these fluorescent polymers exhibited significant thermal properties, along with better solubility in most of common organic solvents for their future application. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 5344–5350, 2006     

Nanoscale ITO/ZnO layer-texturing for high-efficiency InGaN/GaN light emitting diodes

We propose a simple technique to improve the light extraction efficiency of GaN-based light emitting diodes (LEDs) by using nanoscale ITO/ZnO layer-texturing. The surface texturing of the ITO and ZnO layers was performed using a wet chemical etching and a spin-coating process, respectively. It has been found that the light extraction efficiency of the ITO-/ZnO-textured LED was 34.5% greater than that of a conventional LED with a planar ITO, at 20 mA of current injection. A high level of multiple light scattering at the textured surface promoted a high-efficiency in the InGaN/GaN LEDs. In addition, the individual performance of the ITO and ZnO texturing on the LED surface was also investigated. The lowered forward voltage of the ITO/ZnO layer-textured LED indicated this could be a damage-free approach for device fabrication.     

Synthesis and improved luminescence properties of OLED/ZnO hybrid materials

Organic light-emitting diode-zinc oxide (OLED-ZnO) hybrid nanomaterials are successfully prepared by the incorporation of ZnO (a few weight percent) under simple and mild reaction conditions. Three different types of organic light-emitting materials, Fluorescein, Tris(8-hydroxyquinoline) aluminum, and poly(fluoresceinyl terephthalyl benzoate-co-bisphenol A terephthalate) are chosen to prepare the ZnO-incorporated hybrid nanomaterials. Sub-micron to nano-sized OLED-ZnO hybrid materials are synthesized with a distinct morphological feature compared to pure counterparts. ZnO incorporation in the OLED-ZnO hybrids is identified using X-ray diffraction (XRD). The resulting hybrid materials show significant improvement in their photoluminescence intensity, along with a blue shift. The blue shift in photoluminescence emission is attributed to the quantum confinement effect of the incorporated ZnO nanomaterials. The reasons for the improved optical luminescence of OLED-ZnO hybrids are discussed in detail.     

Preparation of exfoliated high‐impact polystyrene/MMT nanocomposites via in situ polymerization under controlling viscosity of the reaction medium

Exfoliated high‐impact polystyrene (HIPS)/montmorillonite (MMT) nanocomposites were prepared via in situ polymerization of styrene in the presence of polybutadiene, using an intercalated cationic radical initiator‐MMT hybrid (organoclay). In the solution polymerization in toluene, the silicate layers of the clay were well exfoliated, due to the low extra‐gallery viscosity that can facilitate the diffusion of styrene monomers into the clay layers during the polymerization. The exfoliated HIPS/MMT nanocomposites were also successfully prepared by controlling the viscosity of the reaction medium with prolong swelling of the organoclay in styrene, prior to bulk polymerization. The HIPS/MMT nanocomposites, obtained from bulk polymerization, exhibited a significant improvement in thermal stability, compared to those obtained from solution polymerization as well as the pure polymer counterparts. POLYM. COMPOS., 2008. © 2007 Society of Plastics Engineers     

Effect of annealing temperature and pH on morphology and optical property of highly dispersible ZnO nanoparticles

Highly dispersible zinc oxide nanoparticles were produced in large quantity via a simple solution method. The effect of temperature and pH impact on as-prepared ZnO nanoparticles with respect to the morphological and optical characteristics has been investigated. The average particle size of ZnO nanoparticles increased with increasing annealing temperature. A sharp UV band-edge emission was observed in as-prepared ZnO nanoparticles with negligibly less intense deep level emission. However, upon annealing at high temperature in air, UV band-edge emission disappears with an evolution of a broad deep level emission in photoluminescence spectra. Similarly, by adjusting the pH of reaction medium from 4 to pH = 8 using ammonium hydroxide solution, particle size gets bigger and bigger leads to red-shift in UV band-edge emission and an appearance of deep level emission peak. At pH = 8, well resolved sharp X-ray diffraction peaks were observed with lower FWHM values due to higher crystallite sizes.     

Polymer solar cells based on inkjet-printed PEDOT:PSS layer

In this article, we have demonstrated solar cell performance of the inkjet-printed PEDOT:PSS layer and the roles of additives in device efficiency. The newly proposed PEDOT:PSS inks with additives of glycerol and surfactant show the improved surface morphology and high conductivity resulting in the enhanced photovoltaic performance. Using the optimized ink formulation of PEDOT:PSS, we have demonstrated a 3.16% efficient solar cell with an inkjet printing.     

Enhancement of light output power in GaN-based light-emitting diodes using indium tin oxide films with nanoporous structures

We fabricated GaN-based light-emitting diodes (LEDs) with a transparent ohmic contact made from nanoporous indium tin oxide (ITO). The nanoporous structures are easily made and controlled using a simple wet etching technique. The transmittance, sheet resistance, and root-mean-square surface roughness of the nanoporous ITO films are correlated strongly with the etch times. On the basis of the experimental values of these parameters, we choose an optimum etch time of 50 s for the fabrication of LEDs. The wall-plug efficiency of the LEDs with nanoporous ITO is increased by 35% compared to conventional LEDs at an injection current of 20 mA. This improvement is attributed to the increase in light scattering at the nanoporous ITO film-to-air interface.     

Preparation and characterization of nano-scale ZnO as a buffer layer for inkjet printing of silver cathode in polymer solar cells

Less-populated and well-isolated ZnO nanorods were prepared from a simple solution method by using polyethylene glycol (PEG) surfactant molecules. The structural and morphological information provided by X-ray diffraction (XRD) and field-emission scanning electron microscopy (FESEM) demonstrated the high purity of the ZnO nanorods that were free from any unknown impurities. Furthermore, annealing treatment was used to increase the length of the ZnO nanorods further at an elevated temperature. This ZnO was used as a buffer layer for polymer solar cells (PSCs) in the device configuration of ITO/ PEDOT:PSS/P3HT-PCBM/ZnO/Ag, in which the Ag cathode was prepared by the inkjet printing method using silver ink. The present study discusses and compares the performance of the devices with and without the ZnO buffer layer.     

Impact of layer thickness and light transmission of ZnO nanomaterials on GaN-based light emitting diodes

Highly transparent ZnO nanomaterials have been successfully dispersed in the form of nanoparticles and nanorods on InGaN/GaN-based surface mounted light emitting diodes (SM-LEDs). An effortless spin-coating technique is employed to disperse the ZnO nanoparticle layers, and a well-known hydrothermal technique is used for growing the ZnO nanorods. The layer thickness and the light transmission at a specific wavelength are the major factors in improving the light output power of the devices. Field emission scanning electron microscope (FESEM) images are used to confirm the uniform dispersion of the ZnO nanostructures on the top of the SM-LEDs. The layer thickness and the level of light transmission at 460 nm are examined from the cross-sectional FESEM images and UV absorption spectra, respectively.     

Synthesis and characterization of polyesters containing fluorescein dye units

Low molecular weight polyesters containing fluorescein units in their backbones were successfully synthesized. These fluorescent polymers showed a high solubility in most of the common organic solvents. The onset degradation temperatures of the polymers were greater than that of fluorescein. The glass transition temperatures were in the range 126-194 °C. The bathochromic fluorescence emissions in organic solutions at high concentrations, which resulted from the aggregation of fluorescein moieties, were prohibited by protecting the OH end groups with benzoyl groups. The polymer powders exhibited a maximum photoluminescence intensity at about 580 nm.